1. Field of the Invention
The present invention relates to an image forming apparatus and a method of adjusting an image and more specifically, to an image forming apparatus and a method of adjusting an image capable of adjusting error caused by a print head.
2. Description of the Related Art
Within an electro-photographic image forming apparatus, a latent image which has an equivalent shape as an original image is formed by irradiating light on a photosensitive body such as a photosensitive drum which is formed in a drum shape, for example. First, the image forming apparatus charges a surface of the photosensitive drum by a charging device, and then irradiates light, which is controlled based on image data, onto the charged photosensitive drum to form an electrostatic latent image on the surface. Subsequently, the image forming apparatus applied toner to the electrostatic latent image formed on the photosensitive drum with a development unit to form a visible toner image. Thereafter, the toner image is transferred to a transfer unit in accordance with a rotation of the photosensitive drum to be electrostatically transferred onto a recording paper or on a middle transfer body. The toner image transferred on the recording paper is fixed.
For an exposure apparatus used in such an image forming apparatus, a laser light scanning apparatus such as a Raster Output Scanner (ROS) in which laser light is scanned in a main scanning direction using a laser diode and a polygon mirror has been used.
However, recently, an exposure apparatus including a light emitting diode print head (LPH: LED Print Head) composed of plural LEDs aligned in the main scanning direction has also been adopted in order to miniaturize the size and decrease an electrical power consumption. The LPH includes an LED array in which plural LED chips composed of plural LEDs aligned in a line are further aligned in the main scanning direction. Further, the LPH generally includes a rod lens array composed of plural aligned rod lenses for focusing light emitted from the corresponding LEDs on a photosensitive body.
The image forming apparatus forms an image on the photosensitive body by controlling the LEDs of the LPH based on input image data to emit light toward the photosensitive body and controlling the rod lens array to form the image with the light on the photosensitive body. With this operation, an electrostatic latent image of a single line extending in the main scanning direction is formed on the photosensitive body. Then, the same operation for forming an image of a next single line extending in the main scanning direction on the photosensitive body is performed after the photosensitive body is rotated with respect to the LPH. By repeating this operation, an entire electrostatic latent image is formed on the photosensitive body in a sub-scanning direction.
For the image forming apparatus including such an LPH, if the irradiation light for a single line extending in the main scanning direction emitted, from the LPH is not in a parallel relationship with an axis direction of the photosensitive body, the electrostatic latent image formed on the photosensitive body includes distortion.
For example, if the irradiation light for a single line extending in the main scanning direction is inclined with respect to the axis direction of the photosensitive body, even when an image of a straight line extending in the main scanning direction is about to be formed, an electrostatic latent image actually formed on the photosensitive body becomes an inclined line with respect to the main scanning direction and the sub-scanning direction.
Further, if the LED chip of a single line extending in the main scanning direction is not aligned on a straight line with respect to the axis direction of the photosensitive body, even when an image of a straight line extending in the main scanning direction is about to be formed, an electrostatic latent image actually formed on the photosensitive body includes a distortion in the sub-scanning direction.
At such a case, when forming a monochromatic image where only a single LPH is used, the result is just an obtained image including distortion. However, when forming a color image by a color printer using a tandem engine, for example, where plural LPHs are used corresponding to plural colors, an obtained image includes distortions of plural colors such that image quality is seriously reduced.
Further, for the image forming apparatus, gradation of image data is converted to be expressed by pixels using image processing such as dithering, and generally, patterns for the image processing such as dithering patterns are designed and prepared not to reduce image quality. However, if the LED chips for a single line extending in the main scanning direction are not aligned on a straight line with respect to the axis direction of the photosensitive body, the influence of dithering is easily reflected to lower the image quality.
Patent Document 1 discloses a technique for correcting the displacement or skew of plural LED chips. In Patent Document 1, an image forming apparatus capable of performing a skew correction in which the plural LEDs aligned in the main scanning direction are divided into plural groups, and timings for lighting the LEDs are shifted for each of the groups. With this skew correction, degradation of image quality due to a skew in the sub-scanning direction or displacement in attaching the LED chips may be reduced.
However, according to the image forming apparatus disclosed in Patent Document 1, there may be error in the skew correction.
FIG. 1 is a view for explaining a method of correcting skew of an LPH and displacement of LED chips in a sub-scanning direction.
The image forming apparatus includes sensors for detecting an amount of skew caused by the LPH. The image forming apparatus, using the LPH, forms alignment marks 200 and 202 on a recording paper 210 (or on a middle transfer body) for detecting the amount of skew caused by the LPH and detects the amount of skew of the LPH in the sub-scanning direction. The amount of skew means a shift amount between timings at which the alignment marks 200 and 202 are detected by the sensors of the image forming apparatus. For the example shown in FIG. 1, the amount of skew is expressed as Δa.
However, some of the LED chips included in the LPH may also be shifted with respect to other LED chips within the LPH. Therefore, it is necessary for the image forming apparatus to correct such displacement of the LED chips as well. For the example shown in FIG. 1, the LED chip “n+2” is shifted by Δb in the forward direction of the sub-scanning direction (shown as “SUB”) with respect to other LED chips “n”, “n+1” and “n+3”. Thus, the image forming apparatus is necessary to correct the skew and the displacement.
However, recently, as image forming apparatuses are becoming able to form high resolution images, the amount of skew Δa or the amount of displacement Δb becomes very small to be smaller than the minimum shifting amount for which the image forming apparatus is capable of correcting. Therefore, for the amount of skew Δa, plural of the LED chips divided into the same block are shifted for an amount corresponding to the minimum shifting amount in the sub-scanning direction. For the amount of displacement Δb, each of the LED chips is also shifted for the amount corresponding to the minimum shifting amount. Thus, by performing both the skew correction and the displacement correction where the amount of skew and the amount of displacement are both added, errors may be added as well. In other words, errors are larger than in the case where only skew correction or only displacement correction is performed.
If the errors are added, the image quality is drastically lowered. For example, by performing image processing such as dithering, due to the regularity of the dithering patterns, relatively large degradation of image quality may occur.